Abstracts

Rationale: The ability of MEG for localizing focal epileptic seizure onset bases on the fact that ictal onset zone is a frequent generator of interictal discharges. It has been well validated by comparing MEG results with intracranial EEG (iEEG) findings as clinical gold standard. However, in cases with diffuse/non-lateralizing ictal EEG and non-focal MRI, invasive evaluations are typically not considered and surgical treatment is usually discouraged. In those cases, the lateralization/localization made by MEG for epileptic focus cannot be tested due to lack of invasive monitoring. In our center, some patients with non-lateralizing seizures but with suspected focal epilepsy underwent bilateral iEEG monitoring for further localization of seizure onset. Under these circumstances, we had the opportunity to evaluate the contribution of MEG to lateralize/localize ictal onset zone in cases with diffuse/non-focal scalp EEG.Methods: We retrospective reviewed the data from 20 consecutive patients who underwent pre-surgical MEG evaluation and bilateral iEEG monitoring because of discordant or non-lateralizing seizure localization. Pre-surgical evaluation included scalp video EEG, 3T MRI, FDG-PET, SPECT and MEG. Broad bilateral craniotomy was carried out for extensive coverage of subdural electrodes. The ictal onset zone was identified by prolonged bilateral iEEG monitoring. MEG-derived sources of interictal epileptiform discharges were calculated at the onset of focal and generalized discharges and later clustered to define the main irritative zone-s. The MEG-derived dipoles were then co-registered with the 3D MRI and implanted electrodes to facilitate comparison of MEG estimates with ictal onset zone identified by bilateral iEEG findings. The degree of agreement in lateralization and localization between bilateral iEEG and MEG was determined on a case by case basis.Results: 20 patients consisted of 10 male and 10 female, aged from 5 months to 35 years. Their scalp video EEG findings mostly showed non-lateralizing ictal and inter-ictal epileptic discharges but with hints of suspected focal onset. Regarding their MRI findings, 3 were negative, 8 had bilateral findings, and the rest 9 had inconclusive unilateral findings. According to their bilateral iEEG findings, among 20 patients, 12 had unilateral focal ictal onset and the rest of 6 had bilateral independent ictal onset and 2 had non-lateralizing ictal onset. MEG findings successfully lateralized 8 out of the 12 patients. In these 8 patients, 6 patients can be further correctly localized by MEG findings.Conclusions: Considering that MEG is based on short (under 60 minutes) recordings of interictal activity, our results validated by bilateral iEEG indicated that MEG made a significant contribution to the lateralization/localization in cases where the ictal scalp EEG recordings were inconclusive. This information might impact the definition of non-lateralizing seizure based on scalp EEG and further surgical decision.